Macro to start autofill from the last cell

Hello,
I been trying to get this code to work, but I am not able to define the last cell from where autofill should start, Could someone help me on this?

Dim a As Long
    For a = 25 To 72
        If Range("A" & a).Value = "" And Range("B" & a) <> "" Then
            'what to do if cell is empty
            Range("A" & a) = "AK"
               Dim lastrow As Long
    lastrow = Worksheets("sheet1").Range("B65536").End(xlUp).Row
    With Worksheets("Sheet1").Range("A25")
        .autofill Destination:=Range("A25:A" & lastrow&) "WHAT CHANGES SHOULD I MAKE HERE SO THAT INSTEAD OF CELL A25 IT WILL TAKE THE CELL VALUE OF RANGE("A" & a)"
    End With
            Exit For
        End If
    Next a

Thanks,
Aadesh Gandhi